This invention relates to the preparation of aromatic heterocyclic polymers. More specifically, it pertains to a process for the formation of rigid-rod aromatic heterocyclic polymers.
Aromatic heterocyclic polymers exhibit excellent tensile strength and thermal stability and are useful for military, aerospace and other applications requiring rigid and environmentally stable materials. U.S. Pat. Nos. 4,533,692; 4,533,693; 4,533,724 and 4,423,202 describe a method of producing heterocyclic aromatic polymers in a polyphosphoric acid solvent. This method basically involves the addition of a monomer to polyphosphoric acid with subsequent heating and optionally a reduction in pressure. A second monomer and phosphorous pentoxide are then added to the mixture to promote polymerization. In order to form a hetercyclic polymer, at least one monomer must be amino-substituted and is usually in the form of a hydrohalide salt. The high viscosity of the polyphosphoric acid hinders the release of hydrohalide from the amino-substituted monomer and prolongs the process over a period of several days. The highly viscous polyphosphoric acid also causes foaming and delays the formation of a homogeneous solution.
In view of the problems associated with current methods of aromatic heterocyclic rigid-rod polymer production, an improved process is needed that would minimize reaction time, reduce undesirable foaming and hasten the formation of a homogeneous solution.